feat(backend): implement multi-agent thinking framework skeleton

Added orchestrator.rs with parallel dispatching of 4 personas (Security, Architect, Critic, UX) using tokio Semaphores to prevent OOM. Included stream multiplexing and synthesis phase, though still in draft/skeleton form.

crates/api-contracts/src/chat.rs

@@ -31,6 +31,8 @@ pub struct ChatCompletionRequest {
     pub parallel_tool_calls: Option<bool>,
     #[serde(default)]
     pub stream_options: Option<StreamOptions>,
+    #[serde(default)]
+    pub agentic_mode: bool,
 }
 
 fn default_max_tokens() -> u32 {
@@ -116,6 +118,12 @@ pub struct ChatCompletionChunk {
     pub choices: Vec<ChunkChoice>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub usage: Option<Usage>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub source: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub expert_id: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub expert_name: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize)]